Superabsorbent cellulosic fiber

ABSTRACT

A modified cellulosic fiber having superabsorbent properties is described. The modified fiber of the invention has a fibrous structure substantially identical to the cellulosic fiber from which it is derived. The modified fiber is a water-swellable, water-insoluble fiber that substantially retains its fibrous structure in its expanded, water-swelled state. The modified fiber is a sulfated and crosslinked cellulosic fiber having a liquid absorption capacity of at least about 4 g/g. In one embodiment, the modified fiber is an individual, crosslinked, sulfated cellulosic fiber. In another aspects, the invention provides a rollgood that includes the modified fiber, absorbent composites and articles that include the modified fiber, and methods for making the modified cellulosic fiber.

FIELD OF THE INVENTION

[0001] The present invention relates to a modified cellulosic fiberhaving superabsorbent properties and, more particularly, to acrosslinked and sulfated cellulosic fiber having a structuresubstantially identical to the fiber from which it is derived.

BACKGROUND OF THE INVENTION

[0002] Personal care absorbent products, such as infant diapers, adultincontinent pads, and feminine care products, typically contain anabsorbent core that includes superabsorbent in a fibrous matrix.Superabsorbents are water-swellable, generally water-insoluble absorbentmaterials having a liquid absorbent capacity of at least about 10,preferably of about 20, and often up to about 100 times their weight inwater. While the core's liquid retention or storage capacity is due inlarge part to the superabsorbent. the core's fibrous matrix provides theessential functions of liquid wicking, pad strength and integrity, andsome amount of absorbency under load. These desirable properties areattributable to the fact that the matrix includes cellulosic fibers,typically wood pulp fluff in fiber form.

[0003] For personal care absorbent products, U.S. southern pine fluffpulp is used almost exclusively and is recognized worldwide as thepreferred fiber for absorbent products. The preference is based on thefluff pulp's advantageous high fiber length (about 2.8 mm) and itsrelative ease of processing from a wetlaid pulp sheet to an airlaid web.However, these fluff pulp fibers can absorb only about 2-3 g/g of liquid(e.g.. water or bodily fluids) within the fibers' cell walls. Most ofthe fibers' liquid holding capacity resides in the interstices betweenfibers. For this reason, a fibrous matrix readily releases acquiredliquid on application of pressure. The tendency to release acquiredliquid can result in significant skin wetness during use of an absorbentproduct that includes a core formed exclusively from cellulosic fibers.Such products also tend to leak acquired liquid because liquid is noteffectively retained in such a fibrous absorbent core.

[0004] The inclusion of absorbent materials in a fibrous matrix andtheir incorporation into personal care products is known. Theincorporation of superabsorbent materials into these products has hadthe effect of reducing the products' overall bulk while at the same timeincreasing its liquid absorbent capacity and enhancing skin dryness forthe products' wearers.

[0005] A variety of materials have been described for use as absorbentmaterials in personal care products. Included among these materials arenatural-based materials such as agar. pectin. gums. carboxyalkyl starchand carboxyalkyl cellulosic fiber, such as carboxymethyl cellulose. aswell as synthetic materials such as polyacrylates. polyacrylamides, andhydrolyzed polyacrylonitriles. Although natural-based absorbingmaterials are well known, these materials have not gained wide usage inpersonal care products because of their relatively inferior absorbentproperties compared to synthetic absorbent materials such aspolyacrylates. The relatively high cost of these materials has alsoprecluded their use in consumer absorbent products. Furthermore. manynatural-based materials tend to form soft, gelatinous masses whenswollen with a liquid. The presence of such gelatinous masses in aproduct's core tends to limit liquid transport and distribution withinthe core and prevents subsequent liquid insults from being efficientlyand effectively absorbed by the product.

[0006] In contrast to the natural-based absorbents. synthetic absorbentmaterials are generally capable of absorbing large quantities of liquidwhile maintaining a relatively non-gelatinous form. Synthetic absorbentmaterials, often referred to as superabsorbent polymers (SAP), have beenincorporated into absorbent articles to provide higher absorbency underpressure and higher absorbency per gram of absorbent material.Superabsorbent polymers are generally supplied as particles having adiameter in the range from about 20-800 microns. Due to their highabsorbent capacity under load, absorbent products that includesuperabsorbent polymer particles provide the benefit of skin dryness.Because superabsorbent polymer particles absorb about 30 times theirweight in liquid under load, these particles provide the furthersignificant advantages of thinness and wearer comfort. In addition,superabsorbent polymer particles are about half the cost per gram ofliquid absorbed under load compared to fluff pulp fibers. For thesereasons it is not surprising that there is a growing trend toward highersuperabsorbent particle levels and reduced levels of fluff pulp inconsumer absorbent products. In fact, some infant diapers include 60 to70 percent by weight superabsorbent polymer in their liquid storagecore. From a cost perspective, a storage core made from 100 percentsuperabsorbent particles is desirable. However. as noted above, such acore would fail to function satisfactorily due to the absence of anysignificant liquid wicking and distribution of acquired liquidthroughout the core. Furthermore, such a core would also lack strengthto retain its wet and/or dry structure, shape, and integrity.

[0007] Cellulosic fibers provide absorbent products with criticalfunctionality that has, to date, not been duplicated by particulatesuperabsorbent polymers. Superabsorbent materials have been introducedin synthetic fiber form seeking to provide a material having thefunctionality of both fiber and superabsorbent polymer particle.However, these superabsorbent fibers are difficult to process comparedto fluff pulp fibers and do not blend well with fluff pulp fibers.Furthermore, synthetic superabsorbent fibers are significantly moreexpensive than superabsorbent polymer particles and, as a result, havenot competed effectively for high volume use in personal care absorbentproducts.

[0008] Cellulosic fibers have also been rendered highly absorptive bychemical modification to include ionic groups such as carboxylic acid,sulfonic acid, and quaternary ammonium groups that impart waterswellability to the fiber. Although some of these modified cellulosicmaterials are soluble in water, some are water- insoluble. However, noneof these highly absorptive modified cellulosic materials possess thestructure of a pulp fiber, rather, these modified cellulosic materialsare typically granular or have a regenerated fibril form.

[0009] A need exists for a highly absorbent material suitable for use inpersonal care absorbent products. the absorbent material havingabsorptive properties similar to synthetic. highly absorptive materialsand at the same time offering the advantages of liquid wicking anddistribution associated with fluff pulp fibers. Accordingly, there is aneed for a fibrous superabsorbent that combines the advantageous liquidstorage capacity of superabsorbent polymers and the advantageous liquidwicking of fluff pulp fibers. Ideally, the fibrous superabsorbent iseconomically viable for use in personal care absorbent products. Thepresent invention seeks to fulfill these needs and provides furtherrelated advantages.

SUMMARY OF THE INVENTION

[0010] In one aspect, the present invention provides a modifiedcellulosic fiber having superabsorbent properties. The modified fiberformed in accordance with the present invention has a fibrous structuresubstantially identical to the cellulosic fiber from which it isderived. More importantly, the modified fiber is a water-swellable,water-insoluble fiber that substantially retains its fibrous structurein its expanded, water-swelled state. The modified fiber is a sulfatedand crosslinked cellulosic fiber having a liquid absorption capacity ofat least about 4 g/g. In one embodiment, the modified fiber is anindividual, crosslinked, sulfated cellulosic fiber. In anotherembodiment, the invention provides a roligood that includes the modifiedfiber. In one embodiment, the rollgood includes other materials such asfibrous, binder, and absorbent materials. In another embodiment, theroilgood can be directly inserted as an absorbent core into an absorbentarticle.

[0011] In another aspect of the invention, methods for forming themodified cellulosic fiber are provided. In one embodiment of the method,a sulfated cellulosic fiber is crosslinked to an extent sufficient torender the fiber substantially insoluble in water. In anotherembodiment, a crosslinked cellulosic fiber is sulfated to provide themodified fiber. The sulfated cellulosic fiber can be prepared byreacting the fiber with sulfuric acid in an organic solvent.

[0012] In others aspects, the invention provides methods for using themodified fiber and absorbent composites and articles incorporating themodified fiber are also provided. In one embodiment. the inventionprovides an absorbent core having a liquid capacity of at least about 22g/g. The absorbent core can be advantageously incorporated into anabsorbent article.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The foregoing aspects and many of the attendant advantages ofthis invention will become more readily appreciated as the same becomebetter understood by reference to the following detailed description,when taken in conjunction with the accompanying drawings, wherein:

[0014] FIGS. 1A-C are scanning electron microscope (SEM) photographs ofrepresentative fluff pulp fibers (bleached kraft southern pine fiberscommercially available from Weyerhaeuser Company under the designationNB416) at 100× magnification (FIG. 1A), at 300× magnification (FIG. 1B),and at 1000× magnification (FIG. 1C);

[0015] FIGS. 2A-C are SEM photographs of representative modified fibersformed in accordance with the present invention from bleached kraftsouthern pine fibers (NB416) at 100× magnification (FIG. 2A), at 300×magnification (FIG. 2B). and at 1000× magnification (FIG. 2C);

[0016]FIGS. 3A and 3B are optical microscope photographs ofrepresentative modified fibers formed in accordance with the presentinvention, FIG. 3A illustrates modified fibers before contact with waterand FIG. 3B illustrates modified fibers after contact with water; and

[0017]FIG. 4 is a graph illustrating the absorbent capacity forrepresentative modified fibers formed in accordance with the presentinvention as a function of weight percent crosslinking applied to thefibers and sulfation reaction time (25 minutes, +; 35 minutes, ▪; 45minutes, Δ).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0018] In one aspect, the present invention provides a modifiedcellulosic fiber having superabsorbent properties. The modified fiberformed in accordance with the present invention has a fibrous structuresubstantially identical to the cellulosic fiber from which it isderived. More importantly. the modified fiber is a water-swellable,water-insoluble fiber that substantially retains its fibrous structurein its expanded, water-swelled state. The cellulosic fiber formed inaccordance with the invention is modified cellulosic fiber that has beensulfated and crosslinked. Water swellability is imparted to thecellulosic fiber through sulfation and intrafiber crosslinking rendersthe cellulosic fiber substantially insoluble in water. The modifiedcellulosic fiber has a degree of sulfate group substitution effective toprovide advantageous water swellability. The modified cellulosic fiberis crosslinked to an extent sufficient to render the fiber waterinsoluble. The modified cellulosic fiber has a liquid absorptioncapacity that is increased compared to unmodified fluff pulp fibers. Themodified fibers have a liquid absorption capacity of at least about 4g/g.

[0019] Cellulosic fibers suitable for use in forming the modified fiberof the present invention are substantially water-insoluble and nothighly water-swellable. After sulfation and crosslinking in accordancewith the present invention, the resulting modified fiber has the desiredabsorbency characteristics. is water-swellable and water-insoluble, andsubstantially retains the fibrous structure of the cellulosic fiber fromwhich it is derived.

[0020] The modified fiber of the invention has the structure of a pulpfiber including a cell wall structure. In one embodiment, the modifiedfiber has the structure of a wood pulp fiber. The modified fiberincludes a lumen (i.e., central cavity) surrounded by a wall surfacehaving four concentric layers. In addition to an outermost primary wall(commonly denoted P). the cell wall includes secondary walls (commonlydenoted S1-S3). The secondary walls include an outer layer (S1) adjacentthe primary wall, an inner layer (S3) adjacent the lumen, and a middlelayer (S2) intermediate the outer and inner secondary layers. Themodified fiber's structure also includes long bundles of cellulosicfibrillar structures. referred to as macrofibrils, fibrils,microfibrils, and elementary fibrils, having varying diameters. Thediameter of fibrillar material depends on the extent of fiberprocessing.

[0021] Cellulose is a principal component of delignified cell walls. Forexample, the secondary cell wall can include unbranched cellulose chainshaving a degree of polymerization up to about 17,000. Accordingly, themodified fiber of the invention is primarily cellulosic in nature havingcellulose as its principal chemical component. Cellulose can beconsidered to be a polymer containing repeating anhydroglucose units.The term “anhydroglucose” refers to the repeating unit in cellulose thatis formed by loss of water from glucose on condensation to form thepolymer. The degree of polymerization (DP) for a given cellulosemolecule is the number of anhydroglucose repeating units in themolecule. The DP for a particular cellulose will depend on its sourceand the extent of polymer degradation on processing.

[0022] In addition to cellulose, the modified fiber can includehemicellulose and lignin. While cellulose is a linear polysaccharideformed from glucose, hemicellulose can be either an unbranched orbranched polysaccharide that includes sugars other than glucose. Unlikecellulose and hemicellulose. which are carbohydrate polymers havingrepeating saccharide units, lignin is a highly branched,three-dimensional polymer composed of aromatic units. Lignin isamorphous in structure and not an integral part of the fiber's fibrillarsystem of carbohydrate polymers.

[0023] For native wood fibers, lignin content is greatest in the outerlayers of the cell wall and decreases rapidly to the layer adjacent thelumen. In contrast, cellulose content is lowest in the primary wall andincreases significantly toward the inner fiber regions. Hemicellulosecontent tends to increase gradually from the outer to the inner regionsof the fiber. A description of the chemical-composition and structure ofwood fibers is provided in Pulp and Paper Manufacture, Volume 1. ThePulping of Wood, Second Edition, R. G. MacDonald, Ed., MacGraw-Hill.1969, pages 39-45.

[0024] The chemical composition of the modified fiber of the inventiondepends, in part. on the extent of processing of the cellulosic fiberfrom which the modified fiber is derived. In general, the modified fiberof the invention is derived from a fiber that has been subjected to apulping process (i.e., a pulp fiber). Pulp fibers are produced bypulping processes that seek to separate cellulose from lignin andhemicellulose leaving the cellulose in fiber form. The amount of ligninand hemicellulose remaining in a pulp fiber after pulping will depend onthe nature and extent of the pulping process.

[0025] Thus, the fiber of the invention is a modified pulp fiber thatretains the basic chemical and structural characteristics of a pulpfiber. The modified fiber has a multiwalled macrostructure as describedabove and is composed of primarily of cellulose and can include somehemicellulose and lignin.

[0026] The modified fiber is substantially insoluble in water. As usedherein, a material will be considered to be water-soluble when itsubstantially dissolves in excess water to form a solution, losing itsfiber form and becoming essentially evenly disbursed throughout a watersolution. A sufficiently sulfated cellulosic fiber that is free from asubstantial degree of crosslinking will be water-soluble, whereas themodified cellulosic fiber of the invention, a sulfated and crosslinkedfiber, is water-insoluble.

[0027] The modified fiber is a water-swellable, water-insoluble fiber.As used herein. the term “water-swellable. water-insoluble” refers to amaterial that, when exposed to an excess of an aqueous medium (e.g..bodily fluids such as urine or blood, water, synthetic urine, or 0.9weight percent solution of sodium chloride in water). swells to anequilibrium volume but does not dissolve into solution. Thewater-swellable. water-insoluble modified cellulosic fibers of theinvention retain their original fibrous structure, but in a highlyexpanded state, during liquid absorption and have sufficient structuralintegrity to resist flow and fusion with neighboring materials. Amodified fiber of the invention is effectively crosslinked to besubstantially insoluble in water while being capable of absorbing atleast about 4 times its weight of a 0.9 weight percent solution ofsodium chloride in water under an applied load of about 0.3 pound persquare inch.

[0028] Cellulosic fibers are a starting material for preparing thesuperabsorbent cellulosic fiber product of the invention. Althoughavailable from other sources, suitable cellulosic fibers are derivedprimarily from wood pulp. Suitable wood pulp fibers for use with theinvention can be obtained from well-known chemical processes such as thekraft and sulfite processes, with or without subsequent bleaching. Pulpfibers can also be processed by thermomechanical, chemithermomechanicalmethods, or combinations thereof. Caustic extractive pulp such asTRUCELL, commercially available from Weyerhaeuser Company, is also asuitable wood pulp fiber. The preferred pulp fiber is produced bychemical methods. Ground wood fibers, recycled or secondary wood pulpfibers, and bleached and unbleached wood pulp fibers can be used.Softwoods and hardwoods can be used. Details of the selection of woodpulp fibers are well-known to those skilled in the art. These fibers arecommercially available from a number of companies, includingWeyerhaeuser Company, the assignee of the present invention. Forexample, suitable cellulosic fibers produced from southern pine that areusable with the present invention are available from WeyerhaeuserCompany under the designations CF416, NF405, PL416, FR516, and NB416. Inone embodiment, the cellulosic fiber useful in making the modified fiberof the invention is a southern pine fiber commercially available fromWeyerhaeuser Company under the designation NB416. In other embodiments,the cellulosic fiber can be selected from among a northern softwoodfiber, a eucalyptus fiber, a rye grass fiber. and a cotton fiber.

[0029] Cellulosic fibers having a wide range of degree of polymerizationare suitable for forming the modified cellulosic fiber of the invention.In one embodiment, the cellulosic fiber has a relatively high degree ofpolymerization, greater than about 1000, and in another embodiment,about 1500.

[0030] In one embodiment, the modified fiber has an average lengthgreater than about 1.0 mm. Consequently, the modified fiber is suitablyprepared from fibers having lengths greater than about 1.0 mm. Fibershaving lengths suitable for preparing the modified fiber includesouthern pine, northern softwood. and eucalyptus fibers, the averagelength of which is about 2.8 mm. about 2.0 mm. and about 1.5 mm.respectively. Fibers with average lengths less than about 1.0 mm haverelatively poorer wicking properties and provide composites havingdiminished pad integrity.

[0031] The modified cellulosic fiber of the invention is a sulfatedcellulosic fiber. As used herein, “sulfated cellulosic fiber” refers toa cellulosic fiber that has been sulfated by reaction of a cellulosicfiber with a sulfating agent. It will be appreciated that the term“sulfated cellulosic fiber” includes free acid and salt forms of thesulfated fiber. Suitable metal salts include sodium, potassium, andlithium salt, among others. A sulfated cellulosic fiber can be producedby reacting a sulfating agent with a hydroxyl group of the cellulosicfiber to provide a cellulose sulfate ester (i.e., acarbon-to-oxygen-to-sulfur ester). The sulfated cellulosic fiber formedin accordance with the present invention differs from othersulfur-containing cellulosic compounds in which the sulfate sulfur atomis attached directly to a carbon atom on the cellulose chain as, forexample. in the case of sulfonated cellulose: or cellulosic compounds inwhich the sulfate sulfur atom is attached indirectly to a carbon atom onthe cellulose chain as, for example. in the case of cellulose alkylsulfonates.

[0032] The modified cellulosic fiber of the invention can becharacterized as having an average degree of sulfate group substitutionof from about 0. I to about 2.0. In one embodiment, the modifiedcellulosic fiber has an average degree of sulfate group substitution offrom about 0.2 to about 1.0. In another embodiment, the modifiedcellulosic fiber has an average degree of sulfate group substitution offrom about 0.3 to about 0.5. As used herein, the “average degree ofsulfate group substitution” refers to the average number of moles ofsulfate groups per mole of glucose unit in the modified fiber. It willbe appreciated that the fibers forned in accordance with the presentinvention include a distribution of sulfate modified fibers having anaverage degree of sulfate substitution as noted above.

[0033] A representative method for preparing sulfated fibers isdescribed in Example 1.

[0034] The modified cellulosic fiber of the invention is an intrafibercrosslinked cellulosic fiber. Crosslinked cellulosic fibers and methodsfor their preparation are disclosed in U.S. Pat. Nos. 5,437,418 and5,225,047 issued to Graef et al., expressly incorporated herein byreference.

[0035] Crosslinked fibers can be prepared by treating fibers with acrosslinking agent. Suitable crosslinking agents useful in producing themodified cellulosic fiber are generally soluble in water and/or alcohol.Suitable cellulosic fiber crosslinking agents include aldehyde,dialdehyde, and related derivatives (e.g., formaldehyde, glyoxal,glutaraldehyde, glyceraldehyde), and urea-based formaldehyde additionproducts (e.g., N-methylol compounds). See, for example, U.S. Pat. Nos.3,224,926; 3,241,533; 3,932,209; 4,035,147; 3,756,913; 4,689,118;4,822,453; U.S. Pat. No. 3,440,135, issued to Chung; U.S. Pat. No.4,935,022. issued to Lash et al.; U.S. Pat. No. 4,889,595, issued toHerron et al.; U.S. Pat. No. 3,819,470, issued to Shaw et al., U.S. Pat.No.3,658,613. issued to Steiger et al.; and U.S. Pat. No. 4,853,086.issued to Graef et al., all of which are expressly incorporated hereinby reference in their entirety. Cellulosic fibers can also becrosslinked by carboxylic acid crosslinking agents includingpolycarboxylic acids. U.S. Pat. Nos. 5,137,537; 5,183,707; and5,190,563, describe the use of C2-C9 polycarboxylic acids that containat least three carboxyl groups (e.g., citric acid and oxydisuccinicacid) as crosslinking agents.

[0036] Suitable urea-based crosslinking agents include methylolatedureas, methylolated cyclic ureas, methylolated lower alkyl substitutedcyclic ureas, methylolated dihydroxy cyclic ureas. dihydroxy cyclicureas, and lower alkyl substituted cyclic ureas. Specific preferredurea-based crosslinking agents include dimethylol urea (DMU,bis[N-hydroxymethyl]urea). dimethylolethylene urea (DMEU,1,3-dihydroxymethyl-2-imidazolidinone), dimethyloldihydroxyethylene urea(DMDHU), 1,3-dihydroxymethyl-4,5-dihydroxy-2-imidazolidinone),di-methylolpropylene urea (DMPU). dimethylolhydantoin (DMH),dimethyldihydroxy urea (DMDHU). dihydroxyethylene urea (DHU),4,5-dihydroxy-2-imidazolidinone), and dimethyldihydroxyethylene urea(DMeDHEU, 4,5-dihydroxy-1,3-dimethyl-2-imidazolidinone).

[0037] Suitable polycarboxylic acid crosslinking agents include citricacid, tartaric acid, malic acid, succinic acid, glutaric acid,citraconic acid, itaconic acid. tartrate monosuccinic acid, maleic acid,1,2,3-propane tricarboxylic acid, 1,2,3,4-butanetetracarboxylic acid,all-cis-cyclopentane tetracarboxylic acid, tetrahydrofurantetracarboxylic acid, 1,2,4,5-benzenetetracarboxylic acid, andbenzenehexacarboxylic acid. Other polycarboxylic acids crosslinkingagents include polymeric poly-carboxylic acids such as poly(acrylicacid), poly(methacrylic acid). poly(maleic acid),poly(methylvinylether-co-maleate) copolymer.poly(methylvinylether-co-itaconate) copolymer, copolymers of acrylicacid. and copolymers of maleic acid. The use of polymeric polycarboxylicacid crosslinking agents such as polyacrylic acid polymers, polymaleicacid polymers. copolymers of acrylic acid. and copolymers of maleic acidis described in U.S. Pat. No. 5,998,511, assigned to WeyerhaeuserCompany and expressly incorporated herein by reference in its entirety.

[0038] Other suitable crosslinking agents include diepoxides such as,for example. vinylcyclohexene dioxide, butadiene dioxide, and diglycidylether; sulfones such as, for example, divinyl sulfone,bis(2-hydroxyethyl)sulfone, bis(2-chloroethyl)sulfone, and disodiumtris(β-sulfatoethyl)sulfonium inner salt; and diisocyanates.

[0039] Mixtures and/or blends of crosslinking agents can also be used.

[0040] The crosslinking agent can include a catalyst to accelerate thebonding reaction between the crosslinking agent and cellulosic fiber.Suitable catalysts include acidic salts, such as ammonium chloride,ammonium sulfate, aluminum chloride, magnesium chloride, and alkalimetal salts of phosphorous-containing acids.

[0041] The modified cellulosic fiber of the invention is a crosslinkedcellulosic fiber. The amount of crosslinking agent applied to the fiberis suitably the amount necessary to render the modified fibersubstantially insoluble in water. The amount of crosslinking agentapplied to the cellulosic fiber will depend on the particularcrosslinking agent and is suitably in the range of from about 0.01 toabout 8.0 percent by weight based on the total weight of cellulosicfiber. In one embodiment, the amount of crosslinking agent applied tothe fibers is in the range from about 0.20 to about 5.0 percent byweight based on the total weight of fibers.

[0042] In one embodiment, the crosslinking agent can be applied to thecellulosic fibers as an aqueous alcoholic solution. Water is present inthe solution in an amount sufficient swell the fiber to an extent toallow for crosslinking within the fiber's cell wall. However, thesolution does not include enough water to dissolve the fiber. Suitablealcohols include those alcohols in which the crosslinking agent issoluble and the fiber to be crosslinked (i.e.. unmodified or sulfatedcellulosic fiber) is not. Representative alcohols include alcohols thatinclude from 1 to 5 carbon atoms, for example. methanol, ethanol,n-propanol, isopropanol, n-butanol, isobutanol, s-butanol, andpentanols. In another embodiment, the crosslinking agent can be appliedto the fibers as an ether solution (e.g., diethyl ether).

[0043] It will be appreciated that due to its fiber structure, themodified fiber of the invention can have a distribution of sulfateand/or crosslinking groups along the fiber's length and through thefiber's cell wall. Generally, there can be greater sulfation and/orcrosslinking on or near the fiber surface than at or near the fibercore. Surface crosslinking may be advantageous to improve modified fiberdryness and provide a better balance of total absorbent capacity andsurface dryness. Fiber swelling and soak time can also effect thesulfation and crosslinking gradients. Such gradients may be due to thefiber structure and can be adjusted and optimized through control ofsulfation and/or crosslinking reaction conditions.

[0044] A representative method for crosslinking sulfated fibers isdescribed in Example 2.

[0045] Scanning electron microscope (SEM) photographs of bleached kraftsouthern pine fibers (NB416) at 100×, 300×, and 1000× magnification areillustrated in FIGS. 1A-C. respectively. SEM photographs ofrepresentative modified fibers formed from NB416 fibers in accordancewith the invention at 100×, 300×, and 1000× magnification areillustrated in FIGS. 2A-C. respectively. Referring to FIGS. 1A-C and2A-C, the modified fibers are ribbon-like and are twisted and curled.and have a structure substantially identical to the fiber from whichthey are derived.

[0046] The modified fiber of the invention has a liquid absorbentcapacity of at least about 4 g/g as measured by the centrifuge capacitytest described in Example 3. In one embodiment, the modified fiber has acapacity of at least about 10 g/g. In another embodiment, the fiber hasa capacity of at least about 15 g/g, and in a further embodiment. thefiber has a capacity of at least about 20 g/g. The absorbent capacity ofrepresentative modified fibers formed in accordance with the presentinvention is described in Example 3.

[0047] As noted above, the modified fiber retains the structure of afiber. FIGS. 3A and 3B are optical microscope photographs ofrepresentative modified fibers formed in accordance with the inventionbefore and after contact with water. FIG. 3A shows representativemodified fibers that have not been contacted with water. Referring toFIG. 3A, these fibers are ribbon-like and are twisted and curled. FIG.3B shows representative modified fibers that have been contacted withwater. Referring to FIG. 3B, these swelled fibers have retained theirfiber structure and have expanded diameters that are from about 3 toabout 6 times their original diameter.

[0048] In another aspect of the invention, methods for making acellulosic fiber having superabsorbent properties are provided. In themethods, cellulosic fibers are sulfated and crosslinked to providesuperabsorbent fibers. In one embodiment, cellulosic fibers are sulfatedand then crosslinked. In this method. sulfated cellulosic fibers aretreated with an amount of crosslinking agent sufficient to render theresulting modified cellulosic fibers substantially insoluble in water.In another embodiment. cellulosic fibers are crosslinked then sulfated.In this method. crosslinked cellulosic fibers are sulfated to render theresulting modified cellulosic fibers highly water absorptive. Themodified cellulosic fiber formed by either method is highly waterabsorptive, water-swellable, water-insoluble, and retains the fibrousstructure of the fibers from which it is derived.

[0049] The modified fiber of the invention is a sulfated cellulosicfiber. Sulfated cellulosic fibers can be made by reacting cellulosicfibers (e.g.. cellulosic fibers that are crosslinked or noncrosslinked)with a sulfating agent. Suitable sulfating agents include concentratedsulfuric acid (95-98%), fuming sulfuric acid (oleun), sulfur trioxideand related complexes including sulfur trioxide/dimethylformamide andsulfur trioxide/pyridine complexes, and chlorosulfonic acid, amongothers. In one embodiment, the sulfating agent is concentrated sulfuricacid.

[0050] The sulfating agent is preferably applied to the fibers as asolution in an organic solvent. Suitable organic solvents includealcohols, pyridine, dimethylformamide. acetic acid including glacialacetic acid, and dioxane. In one embodiment, the organic solvent is analcohol having up to about 6 carbon atoms. Suitable alcohols includemethanol, ethanol, n-propanol. isopropanol, n-butanol, isobutanol,s-butanol, pentanols, and hexanols. In one embodiment, the alcohol isselected from among isopropanol and isobutanol.

[0051] The molar ratio of sulfuric acid to alcohol in the solution canbe varied from about 1:1 to about 4:1. In one embodiment, the molarratio of sulfuric acid to alcohol is about 2.4:1, for example, an 80:20(weight/weight) solution of sulfuric acid in isopropanol. The weightratio of sulfuric acid to cellulosic fibers in the sulfation reactioncan be varied from about 5:1 to about 30:1. At low sulfuric acid ratiosthe reaction is slow and incomplete and at high sulfuric acid ratiossignificant cellulose polymer degradation can occur. In one embodiment.the weight ratio of sulfuric acid to pulp fiber is from about 10:1 toabout 25:1. In another embodiment. the weight ratio of sulfuric acid topulp fiber is about 24:1.

[0052] Highly acidic aqueous environments readily degrade cellulosefibers. It has been reported that concentrated sulfuric acid cannot beused to prepare sulfated cellulose because treating cellulose withsulfuric acid results in a soluble product formed from acid hydrolysisof the cellulose backbone by the sulfuric acid. See. WO 96/15137.However, a water-soluble cellulose sulfate has been reportedly preparedfrom an activated cellulose (20 to 30% water) by direct action ofaqueous sulfuric acid or sulfuric acid dissolved in a volatile organicsolvent such as toluene, carbon tetrachloride, or a lower alkanol.“Cellulose Chemistry and Its Applications”, Ed. T. P. Nevell and S. H.Zeronian, Halstead Press. John Wiley and Sons, 1985. page 350.

[0053] Despite the well-known degradation of cellulose in aqueous acidicsolutions, the present invention provides methods for making sulfatedcellulose fibers without significant cellulose hydrolysis. In themethods of the invention. cellulose fiber degradation (i.e., degree ofpolymerization reduction) is substantially avoided by treating cellulosefibers with a sulfating agent in a nonaqueous environment and/or at lowtemperature (e.g., at or below about 4° C.). To further protect againstfiber degradation (e.g., hydrolysis), a dehydrating agent to absorbwater. including water formed during the sulfation reaction, can beadded to the sulfating reaction mixture. Suitable dehydrating agentsinclude, for example, sulfur trioxide, magnesium sulfate. aceticanhydride, and molecular sieves. In one embodiment, cellulosic fibersare reacted with the sulfating agent at a temperature of about 4° C. andboth the cellulosic fibers and the sulfating agent are cooled to about4° C. prior to reaction. In another embodiment, cellulosic fibers,including cooled fibers. are reacted with the sulfating agent in thepresence of a dehydrating agent.

[0054] Depending upon the extent of sulfation desired, the fibers andsulfating agent are reacted for a period of time of from about 10 toabout 60 minutes. Following this reaction period and prior toneutralizing the resulting sulfated fibers, the sulfated fibers areseparated from excess sulfating agent. In one embodiment, the sulfatedfibers are washed with an alcohol prior to neutralization.

[0055] Prior to crosslinking the sulfated cellulosic fibers to providethe modified fibers of the invention, the fibers can be at leastpartially neutralized with a neutralizing agent. The neutralizing agentis suitably soluble in the sulfation solvent. In one embodiment, theneutralizing agent is a base such as, for example, an alkaline base(e.g., lithium, potassium, sodium or calcium hydroxide; lithium,potassium, or sodium acetate). Alternatively. the neutralizing agent caninclude a multivalent metal salt. Suitable metal salts include cerium,magnesium, calcium, zirconium, and aluminum salts such as ammoniumcerium nitrate, magnesium sulfate, magnesium chloride, calcium chloride,zirconium chloride, aluminum chloride, and aluminum sulfate, amongothers. The use of multivalent metal salts as neutralizing agents alsooffers the advantage of intrafiber crosslinking. Thus, through the useof a multivalent metal salt, the sulfated cellulosic fiber can bepartially neutralized and partially crosslinked. Fibers so treated canbe further crosslinked with other crosslinking agents including thosedescribed above.

[0056] The extent of fiber sulfation is dependent on a number ofreaction conditions including reaction time. For example, in a series ofrepresentative sulfation reactions. a 25 minute reaction time provided afiber that included about 3.8 percent by weight sulfur; a 35 minutereaction time provided a fiber that included about 4.9 percent by weightsulfur; and a 45 minute reaction time provided a fiber that includedabout 6.4 percent by weight sulfur. However, in these experiments, theextended sulfation reaction time had an adverse effect on fiber length(i.e.. cellulose hydrolysis occurred under the prolonged reactionconditions). In viscosity experiments, the sulfated fibers produced bythe 25 and 35 minute reaction conditions provide cellulose solutionsclassified as having a Gardner-Holt bubble tube H viscosity (i.e., about200 Centistokes), while the sulfated fibers produced by the 45 minutereaction provided cellulose solutions classified as having C viscosity(i.e., about 85 Centistokes). The results indicate that at extendedreaction times, significant fiber degradation can occur. The absorbentcapacity of modified fibers prepared from these sulfated fibers isdescribed in Example 3.

[0057] A representative method for preparing sulfated fibers isdescribed in Example 1.

[0058] The at least partially neutralized sulfated cellulosic fibers canthen be crosslinked by applying a crosslinking agent to the fibers. Inone embodiment, the crosslinking agent is applied to the fibers as anaqueous alcoholic solution. In general, the crosslinking agent solutionincludes water sufficient to swell but not dissolve the fibers. Aboveabout 95 percent by weight alcohol, the crosslinking agent does notpenetrate the fiber cell wall sufficiently and the result is acrosslinked fiber having nonuniform crosslinking and low absorbentcapacity. Suitably, the aqueous alcoholic solution includes from about10 to about 50 percent by weight water and from about 50 to about 90percent by weight alcohol. In one embodiment, the crosslinking agentsolution is an aqueous ethanol solution (88 percent by weight ethanol).

[0059] After the fibers have been treated with the crosslinking agent,the crosslinking agent is cured by, for example, heating the treatedfibers, to provide intrafiber crosslinked fibers.

[0060] A representative method for crosslinking sulfated fibers isdescribed in Example 2. The method of Example 2 describes crosslinkingsulfated fibers that have been isolated and dried. Alternatively.sulfated fibers formed as described above and in Example 1 may bedirectly crosslinked, after neutralization, without drying the fibers.

[0061] Thus, in one embodiment, the present invention provides a methodfor making cellulosic fibers having superabsorbent properties thatincludes the step of reacting cellulosic fibers with a sulfating agent,at least partially neutralizing the sulfated fibers to provide fiberssuitable for crosslinking, applying a crosslinking agent to the sulfatedfibers, and then curing the crosslinking agent to provide the modifiedfibers.

[0062] It has been discovered that the nature of the modified fiber ofthe present invention can be varied and controlled by the amount ofwater present in the crosslinking reaction. For example, when it isdesirable to produce the modified fiber in individual fiber form,relatively less water is used in the crosslinking reaction. Conversely,when it is desired that the modified fiber be produced as a sheet or web(e.g., rollgood). the crosslinking reaction includes a relativelygreater amount of water. It has been found that water present during thecrosslinking reaction effects bonding between the individual, modifiedfibers. When the water content is sufficiently high in the crosslinkingreaction, interfiber bonding can occur to provide a structure havingsufficient strength and integrity to provide a fibrous web or sheet ofthe modified fiber suitable for the formation of a rollgood. Where it isdesirable to form the modified fiber in individual form, the modifiedfiber can be baled for shipping and subsequent processing.

[0063] Some interfiber bonding and loss of individual fiber structureoccurs when more than about 50 percent by weight water is present in thecrosslinking reaction. Between from about 50 and about 90 percent byweight alcohol, interfiber bonding occurs without the loss of individualfiber structure.

[0064] The method described above can further include other steps tooptimize the production of the modified fibers of the invention. Tofurther assist in preventing fiber hydrolysis during sulfation, thecellulosic fibers can be dried prior to the sulfation reaction. Thefibers can be dried by any one of a number of drying methods includingheating and chemical methods. For example, the fibers can be dried byheating in a drying oven; solvent exchange with a suitable solvent;solvent exchange with a suitable solvent followed by heating; ortreatment with a dehydrating agent such sulfur trioxide or aceticanhydride. Alternatively. a never-dried fiber can be dried by solventexchange using a suitable solvent.

[0065] For effective sulfation, cellulosic fibers, including driedfibers, can be swelled prior to sulfation using a swelling agent.Suitable swelling agents include, for example, water, glacial aceticacid, acetic anhydride, zinc chloride, sulfuric acid, sulfur trioxide,and ammonia. The fibers can be swelled by mixing the fibers with theswelling agent followed by removing excess swelling agent prior toreacting the fibers with the sulfating agent.

[0066] Thus, in another embodiment, the present invention provides amethod for making cellulosic fibers having superabsorbent propertiesthat includes the steps of swelling cellulosic fibers, including dryfibers, with a swelling agent; separating excess swelling agent from theswelled fibers; reacting the swelled fibers with a sulfating agent;separating excess sulfating agent from the fibers; at least partiallyneutralizing the sulfated fibers to provide fibers suitable forcrosslinking; applying a crosslinking agent to the sulfated fibers; andthen curing the crosslinking agent to provide intrafiber crosslinked,sulfated cellulosic fibers.

[0067] In another embodiment, the modified cellulosic fibers of theinvention can be formed by crosslinking then sulfating the cellulosicfibers. In the method, the modified fibers can be prepared by applying acrosslinking agent to cellulosic fibers; curing the crosslinking agentto provide crosslinked fibers; reacting the crosslinked cellulosicfibers with a sulfating agent; at least partially neutralizing thesulfated. crosslinked fibers; and then drying the sulfated, crosslinkedcellulosic fibers.

[0068] The modified fiber of the invention is formed by methods that donot include dissolving the fiber in solution. In this way, the modifiedfiber retains the structure of the fiber from which it is derived. Thestructure of the modified fiber of the invention is in contrast to otherfibrous materials that lack fiber structure and that are prepared byregeneration from solutions (i.e., formed, for example, byprecipitation, from solutions containing dissolved cellulosicmaterials).

[0069] The modified fiber formed in accordance with the presentinvention has superabsorbent properties while, at the same time, has thestructure of the cellulosic pulp fiber from which it is derived. Asnoted above, the modified fiber of the invention can be produced as anindividual fiber or as sheet or web (e.g.. rollgood) of fibers. Thenature of the modified fiber produced depends on the use for which thefiber is ultimately intended.

[0070] The modified fibers can be incorporated into a personal careabsorbent product. The modified fibers can be formed into a compositefor incorporation into a personal care absorbent product. Composites canbe formed from the modified fibers alone or by combining the modifiedfibers with other materials, including fibrous materials, bindermaterials, other absorbent materials, and other materials commonlyemployed in personal care absorbent products. Suitable fibrous materialsinclude synthetic fibers, such as polyester, polypropylene, andbicomponent binding fibers; and cellulosic fibers, such as fluff pulpfibers, crosslinked cellulosic fibers, cotton fibers, and CTMP fibers.Suitable absorbent materials include natural absorbents, such assphagnum moss, and synthetic superabsorbents, such as polyacrylates(e.g., SAPs).

[0071] In one embodiment, the modified fiber is further treated with acompatible material to provide a coated modified fiber. The modifiedfiber can be coated with a variety of materials including those notedabove as well as binders, pH control agents, and odor reducing agents,among others.

[0072] Webs that include the modified fibers can be prepared in any oneof a variety of methods known in the web-forming art. The methodsinclude airlaid and wet forming methods. As noted above, wet-formed websthat include the modified fibers can be formed by, for example, addingwater in an amount sufficient to bond the crosslinked sulfated fibers toan extent sufficient to provide a web with structural integrity. Othermaterials, such as fibrous and absorbent materials, can also be includedin these webs.

[0073] In some instances. when intended for use in a personal careabsorbent product, the rollgood form of the modified fiber is desired.One advantage of the modified fiber in rollgood form is that it can bedirectly incorporated as received by a diaper manufacturer by cuttingthe rollgood into the desired shape and size, and inserting the shapedand sized web into an absorbent article. In this way. the modified fiberin rollgood form can be directly utilized in a diaper manufacturingline. The rollgood containing the modified fiber can also include anyone or more of a variety of other useful materials such as thoseidentified above.

[0074] Absorbent composites derived from or that include the modifiedfibers of the invention can be advantageously incorporated into avariety of absorbent articles such as diapers including disposablediapers and training pants; feminine care products including sanitarynapkins. and pant liners; adult incontinence products; toweling,surgical and dental sponges; bandages: food tray pads: and the like.Thus, in another aspect, the present invention provides absorbentcomposites and absorbent articles that include the modified fiber.

[0075] As noted above, the modified fiber of the invention has a fiberstructure that, like other pulp fibers, provides for liquid wicking.Like superabsorbent materials, the modified fiber has a high liquidabsorbent capacity. Accordingly, the modified fiber can be useful inabsorbent products such as, for example, an infant diaper, where liquidwicking and liquid storage are required. Because of its unique, liquidwicking and capacity properties, the modified fiber can be formed into acomposite and utilized as a storage core in a diaper. Such a core mayonly include the modified fiber. For a modified fiber having anabsorbent capacity of at least about 22 g/g. the resulting core has anabsorbent capacity of at least about 22 g/g. Conventional, commercialdiaper storage cores typically include two components: (1) fluff pulpfibers to wick liquid, and (2) superabsorbent material to store acquiredliquid. The core typically consists of minimally about 25 percent byweight fluff pulp fibers and maximally about 75 percent by weightsuperabsorbent material. Superabsorbent materials generally have anabsorbent capacity of about 28 g/g and fluff pulp fibers generally havean absorbent capacity of about 2 g/g. Therefore, such a core has acapacity of about 22 g/g. Cores prepared from a modified fiber having acapacity of at least about 22 g/g can exceed the performancecharacteristics of conventional absorbent composites. Thus, the modifiedfibers of the invention provide advantages related to the manufacture ofabsorbent cores.

[0076] The following examples are provided for the purposes ofillustrating, not limiting, the present invention.

EXAMPLES Example 1 The Preparation of Sulfated Cellulosic Fibers

[0077] In this example. a representative method for forming sulfatedcellulosic fibers is described.

[0078] Prior to sulfation, the pulp was activated with acetic acid. Tengrams of fiberized bleached kraft southern yellow pine fluff pulp(NB416. Weyerhaeuser Company. Federal Way, Wash.) that had been ovendried at 105° C. was disbursed in 600 mL of glacial acetic acid. Thepulp/acid slurry was then placed in a vacuum chamber and the air wasevacuated. The slurry was allowed to stand under vacuum for 30 minutesafter which time the chamber was repressurized to atmospheric pressure.The slurry was then allowed to stand at ambient conditions for 45minutes before being resubjected to a vacuum for an additional 30minutes. After the second application of a vacuum the slurry was againallowed to stand for 45 minutes at atmospheric pressure. The slurry wasthen poured into a Buchner funnel where the pulp was collected andpressed until the weight of the residual acetic acid was equal to twicethe weight of the oven dry pulp (i.e., total weight of the collectedpulp was 30 g.) The collected pulp was placed inside a plastic bag andcooled to −10° C. in a freezer.

[0079] The sulfation liquor was prepared by mixing 240 g concentratedsulfuric acid with 60 g isopropanol and 0.226 g magnesium sulfate. Theliquor was prepared by pouring isopropanol into a beaker that wasmaintained at 4° C. in an ice bath. Magnesium sulfate was then added tothe isopropanol and the mixture chilled to 4° C. Sulfuric acid wasweighed into a beaker and separately chilled to 9° C. before beingslowly mixed into the isopropanol and magnesium sulfate mixture. Theresulting sulfating liquor was then allowed to cool to 4° C.

[0080] The cooled acetic acid activated pulp (−10° C.) was stirred intothe cooled sulfation liquor (4° C.). The resulting slurry of pulp andsulfation liquor was allowed to react for 35 minutes with constantstirring. After 35 minutes the pulp/sulfation liquor slurry was pouredinto a Buchner funnel and the sulfated pulp was collected and washedover a vacuum with cooled isopropanol (−10° C.). The collected pulp wasthen slurried with cooled isopropanol (−10° C.) in a Waring blender andpoured back into the Buchner funnel where the pulp was again washed withcooled isopropanol (−10° C.).

[0081] The nature and quality of the modified fiber formed in accordancewith the invention can depend on the washing step. First, the acid ispreferably washed from the pulp as quickly as possible to preventcontinued and/or accelerated cellulose degradation. Second, the cooltemperature of the pulp is preferably maintained to prevent cellulosedegradation. Third, the acid is preferably washed from the pulp asthoroughly as possible before neutralization to prevent the formation ofdifficult to remove inorganic salts during the neutralization step.These salts can adversely impact modified fiber absorbency.)

[0082] The washed sulfated pulp was next slurried in cooled isopropanol(−10° C.) and an ethanolic sodium hydroxide solution was added dropwiseuntil the slurry was neutralized. The slurry was then poured into aBuchner funnel where the neutralized sulfated pulp was washed with roomtemperature isopropanol. The neutralized sulfated pulp was then agitatedto remove any inorganic salts that may have been crusted on the fibersurfaces after which the neutralized sulfated pulp was again washed withisopropanol in a Buchner funnel. Finally the collected sulfated pulp wasallowed to air dry.

Example 2 The Preparation of Representative Crosslinked, SulfatedCellulosic Fibers

[0083] In this example, a representative method for forming crosslinked,sulfated cellulosic fibers is described. Sulfated cellulosic fibersprepared as described in Example 1 were crosslinked with arepresentative crosslinking agent.

[0084] A catalyzed urea-formaldehyde system was used to crosslink thesulfated cellulosic fibers. The catalyst included magnesium chloride andthe sodium salt of dodecylbenzenesulfonic acid dissolved in 88%ethanol/water. In addition to its primary function, the catalystsolution served as a diluent for the crosslinking agent. Thecrosslinking agent was obtained by dissolving urea in 37 percent (w/w)aqueous formaldehyde. The crosslinking agent was combined with thecatalyst solution and applied to the sulfated fibers. The treated fiberswere then cured by placing in a 105° C. oven for 60 minutes.

[0085] In the experiment, varying amounts of crosslinking agents wereapplied to the fibers. The amount of crosslinking agent used ranged from1-11 percent of the weight of the sulfated fibers and the amount ofcatalytic diluent used was 250 percent of the weight of the sulfatedfibers. The materials and their amounts used in preparing the catalyticdiluent and crosslinking agent solutions are shown in Table 1 below.TABLE 1 Composition of Catalytic Diluent and Crosslinking AgentSolution. Parts Catalytic Diluent Denatured ethanol 44 Deionized water 6Magnesium chloride heptahydrate 0.214 Dodecylbenzenesulfonic acid,sodium salt 0.4 Crosslinking Agent Solution Urea 15 37% (w/w)Formaldehyde 41

Example 3 The Performance Characteristics of Representative Crosslinked,Sulfated Cellulosic Fibers

[0086] In this example, the performance characteristics ofrepresentative crosslinked. sulfated cellulosic fibers formed inaccordance with the present invention is described. Representativemodified fibers. prepared as described in Examples 1 and 2 above, withvarying levels of crosslinking agent applied to the fibers wereevaluated for absorbent capacity by the total absorptive capacity/teabag gel volume test described below. Modified fiber absorbent capacityas a function of crosslinking agent applied to the fiber is summarizedin Table 2 below.

[0087] The preparation of materials, test procedure, and calculations todetermine absorbent capacity were as follows.

[0088] Preparation of materials:

[0089] 1) Tea bag preparation: unroll tea bag material (Dexter #1 234Theat-sealable tea bag material) and cut cross ways into 6 cm pieces.Fold lengthwise, outside-to-outside. Heatseal edges ⅛ inch with an iron(high setting), leave top end open. Trim excess from top edge to form a6 cm×6 cm bag. Prepare 3 tea bags.

[0090] 2) Label edge with sample identification.

[0091] 3) Preweigh tea bag and record weight (to nearest 0.001 g).

[0092] 4) Weigh 0.200 g sample (nearest 0.001 g) on tared glassine andrecord weight.

[0093] 5) Fill tea bags with modified fiber sample.

[0094] 6) Seal top edge of tea bag ⅛ inch with the iron.

[0095] 7) Weigh and record total weight of tea bag filled with modifiedfiber sample. Store in sealed plastic bag until ready to test.

[0096] Test Procedure:

[0097] 1) Fill container to a depth of at least 2 inch with 1 percent byweight saline solution.

[0098] 2) Hold tea bag horizontally and distribute modified fiber sampleevenly throughout tea bag.

[0099] 3) Lay tea bag on the liquid surface of the saline solution(begin timing) and allow tea bag to wet-out before submerging the teabag (about 10 sec.).

[0100] 4) Soak tea bag for 30 minutes.

[0101] 5) Remove tea bag from the saline solution with tweezers and clipto a drip rack.

[0102] 6) Allow tea bag to hang for 3 minutes.

[0103] 7) Carefully remove tea bag from clip and lightly touch saturatedcorner of tea bag on blotter to remove excess fluid. Weigh tea bag andrecord weight (i.e., drip weight).

[0104] 8) Place tea bag on wall of centrifuge by pressing top edgeagainst the wall. Balance centrifuge by placing the tea bags around thecentrifuge's circumference.

[0105] 9) Centrifuge at 2800 rpm for 75 seconds.

[0106] 10) Remove tea bag from centrifuge, weigh and record tea bagcentrifuged weight.

[0107] Absorbent Centrifuge Capacity Calculation:

[0108] (Net wet weight sample—Net dry weight sample)/Net dry weightsample=g/g capacity

[0109] Net wet weight is the centrifuge weight less the dry weight ofthe tea bag and fiber sample. Net dry weight is the dry weight of thefiber sample.

[0110] The absorbent capacity (g/g), determined as described above, as afunction of sulfation reaction time and crosslinking agent applied tothe fiber for representative modified fibers is summarized in Table 2below and illustrated graphically in FIG. 4. TABLE 2 Modified FiberAbsorbent Capacity: Crosslinking Level and Sulfation Reaction TimeEffect. Crosslinking Centrifuge Capacity (g/g) level (percent) 25 35 45by weight) minute sulfation minute sulfation minute sulfation 1.08 13.012.1 7.0 1.62 15.3 14.6 10.1 1.94 17.2 2.27 15.1 2.48 17.3 2.27 14.718.0 2.97 11.3 3.24 11.9 3.78 8.1 7.9 8.6 4.00 6.6

[0111] As shown in Table 2 and FIG. 4, to a point, absorbent capacityincreases with increasing sulfation. However, at the point wheresulfation results in fiber degradation. absorbent capacity decreases.The results also demonstrate that absorbent capacity also increases withincreasing crosslinking to a point. At higher levels of crosslinking.absorbent capacity decreases.

[0112] While the preferred embodiment of the invention has beenillustrated and described, it will be appreciated that various changescan be made therein without departing from the spirit and scope of theinvention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A modified cellulosicfiber, comprising a sulfated cellulosic fiber crosslinked to an extentto render the fiber substantially insoluble in water.
 2. The fiber ofclaim I having a liquid absorption capacity of at least about 4 g/g. 3.The fiber of claim 1, wherein the average degree of sulfate substitutionis from about 0.1 to about 2.0.
 4. The fiber of claim 1, wherein theaverage degree of sulfate substitution is from about 0.2 to about 1.0.5. The fiber of claim 1, wherein the average degree of sulfatesubstitution is from about 0.3 to about 0.5.
 6. The fiber of claim 1.wherein the cellulosic fiber is a wood pulp fiber.
 7. Individual,water-swellable, water-insoluble, intrafiber crosslinked, sulfatedcellulosic fibers.
 8. The fibers of claim 7 having a liquid absorptioncapacity of at least about 4 g/g.
 9. The fibers of claim 7, wherein theaverage degree of sulfate substitution is from about 0.1 to about 2.0.10. The fibers of claim 7, wherein the average degree of sulfatesubstitution is from about 0.2 to about 1.0.
 11. The fibers of claim 7,wherein the average degree of sulfate substitution is from about 0.3 toabout 0.5.
 12. The fibers of claim 7, wherein the cellulosic fibers arewood pulp fibers.
 13. The fibers of claim 7, wherein the fibers arecrosslinked with a crosslinking agent selected from the group consistingof a urea-based crosslinking agent, a polycarboxylic acid crosslinkingagent, an aldehyde crosslinking agent, a dialdehyde crosslinking agent.and mixtures thereof.
 14. The fibers of claim 13, wherein thecrosslinking agent is applied to the fibers in an amount from about 0.01to about 8.0 percent by weight based on the total weight of fibers. 15.The fibers of claim
 13. wherein the crosslinking agent is applied to thefibers in an amount from about 0.02 to about 5.0 percent bv weight basedon the total weight of fibers.
 16. A rollgood comprising the fibers ofclaims 1 or
 7. 17. The rollgood of claim 16 further comprising anotherfiber.
 18. The rollgood of claim 17, wherein the other fiber is at leastone of fluff pulp fibers. crosslinked cellulosic fibers. cotton fibers.CTMP fibers. and synthetic fibers.
 19. The rollgood of claim 16 furthercomprising an absorbent material.
 20. The rollgood of claim 16 furthercomprising a binder material.
 21. An absorbent article comprising therollgood of claim
 16. 22. An absorbent composite comprising the fibersof claims 1 or
 7. 23. The composite of claim 22 further comprisinganother fiber.
 24. The composite of claim 23, wherein the other fiber isat least one of fluff pulp fibers, crosslinked cellulosic fibers, cottonfibers, CTMP fibers, and synthetic fibers.
 25. An absorbent articlecomprising the fibers of claims 1 or
 7. 26. The article of claim 25,wherein the article is at least one of an infant diaper, an adultincontinence product. and a feminine care product.
 27. An absorbentarticle comprising a liquid pervious topsheet, a liquid imperviousbacksheet attached to the topsheet, and an absorbent member intermediatethe topsheet and backsheet, wherein the absorbent member comprises thefibers of claims 1 or
 7. 28. An absorbent article comprising a liquidpervious topsheet, a liquid impervious backsheet attached to thetopsheet, and an absorbent member intermediate the topsheet andbacksheet, wherein the absorbent member comprises the rollgood of claim16.
 29. A method for making cellulosic fibers, comprising crosslinkingsulfated cellulosic fibers to render the fibers substantiallywater-insoluble.
 30. The method of claim 29, wherein crosslinkingsulfated cellulosic fibers comprises treating sulfated cellulosic fiberswith an amount of a crosslinking agent sufficient to render thecrosslinked fibers substantially water insoluble.
 31. The method ofclaim 30, wherein the amount of crosslinking agent ranges from about0.01 to about 8.0 percent by weight crosslinking agent based on thetotal weight of fibers.
 32. The method of claim 30, wherein thecrosslinking agent is selected from the group consisting of a urea-basedcrosslinking agent, a polycarboxylic acid crosslinking agent, analdehyde crosslinking agent, a dialdehyde crosslinking agent, andmixtures thereof.
 33. The method of claim 30, wherein the crosslinkingagent is applied to the fibers as an aqueous alcoholic solution.
 34. Themethod of claim 29, wherein the sulfated cellulosic fibers have anaverage degree of sulfate substitution of from about 0.1 to about 2.0.35. The method of claim 29 further comprising baling the crosslinked,sulfated fibers.
 36. The method of claim 29 further comprising formingthe crosslinked, sulfated fibers into a rollgood.
 37. A method formaking cellulosic fibers, comprising the steps of: reacting cellulosicfibers with a sulfating agent to provide sulfated fibers; applying acrosslinking agent to the sulfated fibers; and curing the crosslinkingagent to provide crosslinked, sulfated cellulosic fibers.
 38. The methodof claim 37, wherein the sulfating agent comprises sulfuric acid. 39.The method of claim 37, wherein the sulfating agent comprises a solutionof sulfuric acid in an organic solvent.
 40. The method of claim 37,wherein the organic solvent is an alcohol is selected from the groupconsisting of isopropanol, propanol, and butanol.
 41. The method ofclaim 40, wherein the ratio of sulfuric acid to alcohol is about 2.4:1by mole.
 42. The method of claim 37 further comprising treating thesulfated fibers with a neutralizing agent prior to crosslinking.
 43. Themethod of claim 42, wherein the neutralizing agent comprises a base. 44.The method of claim 42, wherein the neutralizing agent comprises amultivalent metal salt.
 45. The method of claim 44, wherein the metalsalt is selected from the group consisting of cerium nitrate, magnesiumsulfate, and aluminum sulfate.
 46. The method of claim 37, wherein thecrosslinking agent is selected from the group consisting of a urea-basedcrosslinking agent, a polycarboxylic acid crosslinking agent, analdehyde crosslinking agent, a dialdehyde crosslinking agent, andmixtures thereof.
 47. The method of claim 37, wherein the crosslinkingagent is applied to the fibers as an aqueous alcoholic solution.
 48. Themethod of claim 37, wherein the cellulosic fibers are reacted with thesulfating agent at a temperature of about 4° C.
 49. The method of claim37 further comprising swelling the fibers prior to reacting the fiberswith the sulfating agent.
 50. The method of claim 49, wherein swellingthe fibers comprises treating the fibers with a swelling agent selectedfrom the group consisting of acetic acid. acetic anhydride, and mixturesthereof.
 51. The method of claim 50 further comprising removing excessswelling agent prior to reacting the fibers with the sulfating agent.52. The method of claim 37, wherein reacting the fibers with a sulfatingagent comprises adding the fibers to an alcoholic solution of thesulfating agent.
 53. The method of claim 52, wherein the fibers arecooled to about 4° C. prior to adding the fibers to the alcoholicsolution of the sulfating agent.
 54. The method of claim 52, wherein thealcoholic solution of the sulfating agent is cooled to about 4° C. priorto the addition.
 55. The method of claim 37, wherein the sulfating agentis reacted with the fibers at a temperature of about 4° C.
 56. Themethod of claim 42 further comprising separating the sulfated fibersfrom excess sulfating agent prior to treating the sulfated fibers withthe neutralizing agent.
 57. The method of claim 42 further comprisingwashing the sulfated fibers with alcohol solution prior to treating thesulfated fibers with the neutralizing agent.
 58. The method of claim 37,wherein the cellulosic fibers further comprise magnesium sulfate. 59.The method of claim 37, wherein the sulfating agent further comprisesmagnesium sulfate.
 60. The method of claim 40, wherein the alcoholicsolution of the sulfating agent further comprises magnesium sulfate. 61.A method for making cellulosic fibers, comprising the steps of: swellingdry cellulosic fibers with a swelling agent to provide swelled fibers;separating excess swelling agent from the swelled fibers; reactingswelled cellulosic fibers with a sulfating agent to provide sulfatedfibers; separating excess sulfating agent from the sulfated fibers;treating the sulfated fibers with a neutralizing agent to provide fiberssuitable for crosslinking; applying a crosslinking agent to the sulfatedfibers; and curing the crosslinking agent to provide crosslinked,sulfated cellulosic fibers.
 62. The method of claim 61, wherein the drycellulosic fibers comprise never-dried cellulosic fibers solventexchanged with an alcohol.
 63. The method of claim 61, wherein theswelling agent is selected from the group consisting of acetic acid,acetic anhydride, and mixtures thereof.
 64. The method of claim 61,wherein the sulfating agent comprises sulfuric acid.
 65. The method ofclaim 61, wherein the sulfating agent comprises a solution of sulfuricacid in an alcohol.
 66. The method of claim 65, wherein the ratio ofsulfuric acid to alcohol is about 2.4:1.
 67. The method of claim 65,wherein the alcohol comprises isopropanol.
 68. The method of claim 61,wherein the neutralizing agent comprises sodium hydroxide.
 69. Themethod of claim 61, wherein the crosslinking agent is applied to thefibers as an aqueous alcoholic solution.
 70. The method of claim 61,wherein the cellulosic fibers are reacted with the sulfating agent at atemperature of about 4° C.
 71. The method of claim 61, wherein thecellulosic fibers are reacted with the sulfating agent for a period oftime from about 10 to about 60 minutes.
 72. The method of claim 61further comprising washing the sulfated fibers with an alcohol solutionprior to treating the sulfated fibers with the neutralizing agent. 73.The method of claim 61, wherein the sulfating agent further comprisesmagnesium sulfate.
 74. The product obtainable by the process of claim29.
 75. The product obtainable by the process of claim
 37. 76. Theproduct obtainable by the process of claim
 61. 77. An absorbent core foran absorbent article, comprising a sulfated cellulosic fiber crosslinkedto an extent to render the fiber substantially insoluble in water,wherein the core has a liquid absorption capacity of at least about 22g/g.
 78. An absorbent core for an absorbent article, comprisingindividual, water-swellable, water-insoluble, intrafiber crosslinked,sulfated cellulosic fibers, wherein the core has a liquid absorptioncapacity of at least about 22 g/g.